Adaptive air interface waveform

ABSTRACT

In one embodiment, a method for generating an adaptive air interface waveform includes generating a waveform that includes a variable carrier frequency and variable bandwidth signal. The variable bandwidth signal includes one or more subcarriers that are dynamically placeable over a range of frequencies, and each subcarrier is separately modulated according to a direct sequence (DS) spread spectrum (SS) technique. The waveform has an embedded pilot usable to optimize one or more spectrum efficiencies of the waveform. A modulation constellation, a code rate, and a code length of the generated waveform are adapted according to an available spectrum and one or more sub-carrier conditions.

RELATED APPLICATION

[0001] This application claims the benefit, under 35 U.S.C. § 119(e), ofU.S. Provisional Application No. 60/375,855, filed Apr. 25, 2002.

TECHNICAL FIELD OF THE INVENTION

[0002] This invention relates to wireless communication and moreparticularly to an adaptive air interface waveform.

BACKGROUND OF THE INVENTION

[0003] Current wireless communication systems do not adjust well todynamic changes in the electromagnetic spectrum. As a result, thesesystems tend to provide a relatively low quality of service. As demandfor high-bandwidth services increases, this problem will likely worsen.

[0004] Prior attempts to improve the ability of wireless communicationsystems to adjust to dynamic changes in the electromagnetic spectrumhave focused on adaptation in a subset of dimensions available at aparticular point in time. Data rates and processing gains have beenmodified to adapt specific waveforms, such as spread spectrum modulatedsignals, to a particular communication link condition. Variouserror-correction coding techniques with various parameters have beenapplied to a particular frequency assignment. Frequency adaptationtechniques have been used in high frequency (HF) ranges. Frequencyadaptation techniques have also been used in communication systems, suchas wireless local area networks (WLANs), in which an open frequency isselected after a relatively slow seat for an open frequency.

[0005] Cellular communication systems typically operate at assignedchannel frequencies. Slow assignments can use frequency divisionmultiple access (FDMA) techniques. Adaptive modulation techniques havebeen investigated, but have been more or less limited to changing one ormore parameters in a particular modulation scheme. Spectrum use can varyconsiderably throughout the world, which often necessitates a complexspectrum assignment process. Reallocation of bandwidth as a result ofgrowth in commercial wireless markets could necessitate even morecomplex spectrum assignment processes in the future. In current wirelesscommunication systems, one or more frequencies are statically assignedto a communication and sensor system (such as a radar system) withoutfrequency overlap between the communication and sensor system and one ormore other communication and sensor systems and with large spatialseparation to prevent harmful interference between the communication andsensor system and one or more other communication and sensor systems.

SUMMARY OF THE INVENTION

[0006] Particular embodiments of the present invention may reduce oreliminate disadvantages and problems traditionally associated withwireless communication.

[0007] In one embodiment of the present invention, a method forgenerating an adaptive air interface waveform includes generating awaveform that includes a variable carrier frequency and variablebandwidth signal. The variable bandwidth signal includes one or moresubcarriers that are dynamically placeable over a range of frequencies,and each subcarrier is separately modulated according to a directsequence (DS) spread spectrum (SS) technique. The waveform has anembedded pilot usable to optimize one or more spectrum efficiencies ofthe waveform. A modulation constellation, a code rate, and a code lengthof the generated waveform are adapted according to an available spectrumand one or more sub-carrier conditions.

[0008] Particular embodiments of the present invention provide one ormore advantages. In particular embodiments, dynamic adaptation inmultiple parameters provides one or more performance options forwireless communication systems. In particular embodiments, the multipleparameters include adaptation in time, adaptation in power, variablebandwidth, variable data rate, variable modulation and coding, andspatial adaptation.

[0009] Particular embodiments provide a waveform that can adapt to anenvironment in multiple dimensions of available signal space. Inparticular embodiments, as an example, the signal space includesfrequency, time, power, modulation, code, and spatial domain. Particularembodiments provide a waveform and a mechanism for selecting one or moreparameters of the waveform and changing the waveform to adapt to one ormore communication networks, one or more communication links, or one ormore user requirements. Particular embodiments provide intelligentselection of multiple dimensions of an adaptation space, which caninclude frequency, modulation scheme and related parameters, codingscheme and related parameters, and data rates. Particular embodimentscan provide a waveform optimized according to one or more linkconditions. In particular embodiments, a modulation scheme can formmultiple constellations and spatially adapt to transmission times. Inparticular embodiments, modulation uses a multi-carrier code divisionmultiple access (MC-CDMA) CDMA) scheme according to which one or moreindividual carriers are independently modulated and coded according tothe adaptation of the individual carriers to one or more communicationlinks. In particular embodiments, adaptation to a communication link ismore or less subject to one or more requirements associated with changesin data rates and frequency over time. In particular embodiments, one ormore frequencies can be blocked or emphasized (effectively providingpower control at each frequency), which can enable use of noncontiguousfrequency sub-bands. In particular embodiments, a particular modulationand coding scheme is selected for a particular sub-band. In particularembodiments, a heteromorphic waveform can be morphed to one or morewireless communication resources (such as one or more frequency bands).In particular embodiments, frequency, modulation type and relatedparameter, coding type and related parameter, time, space, power,bandwidth, and processing are analyzed to provide relatively fastadaptation to time-varying channel conditions.

[0010] Particular embodiments provide an adaptable waveform for multiplewireless applications, such as applications for selecting multipledimensions of an adaptation space and applications for estimatingchannel characteristics. In particular embodiments, power is controlledat frequencies in a waveform. In particular embodiments, noncontiguousfrequency sub-bands are generated. In particular embodiments, apreferred channel organization is identified and selected. In particularembodiments, a preferred modulation and coding technique is selectedaccording to one or more requirements associated with data rate andquality of service.

[0011] In particular embodiments, a spectrum-aware heteromorphicwaveform that dynamically adapts to use available holes in a spectrumdefined by frequency, space, and time enables shared use of commonspectra. In particular embodiments, simultaneous adaptation of multiplewaveform parameters enables more or less assured communication, whilesuppressing mutual harmful interference. Particular embodiments providedynamic spectral assignment techniques that increase spectrumutilization by a factor of twenty.

[0012] Particular embodiments provide quick-response adaptivemulti-carrier reorganization using one or more suitable availablefrequencies. Particular embodiments provide a signal design thatincludes a pilot for real-time sub-carrier channel estimation to more orless optimize waveform parameters and includes fast signal acquisitionfor transmission bursts. Particular embodiments provide one or moreadaptive bandwidth-efficient code-modulation schemes with more or lesssimultaneous multi-dimensional variability with respect to multiplesub-carriers. Particular embodiments provide fast-reaction capability toquickly release channel usage and dynamically reconfigure hybridmultiple-access techniques.

[0013] Particular embodiments provide a single adaptable waveform thatcan work in multiple applications, such as WLAN applications andcellular applications. Particular embodiments provide a useful airinterface that works in heterogeneous networks and can operate at datarates ranging from approximately 100 Mbps to 1 Gbps. A networkenvironment could include a cellular macro environment, a micro-picocellular environment, a WLAN or similar environment. A networkenvironment could include one or more flexible architectures, such ascellular, centralized, ad hoc, and hybrid architectures. Particularembodiments support services and applications that have relatively highrates of data transmission. Particular embodiments automatically operatein gaps (or holes) in spectrum use. A hole can include multipledimensions, such as time, frequency, and space.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] To provide a more complete understanding of the present inventionand the features and advantages thereof, reference is made to thefollowing description, taken in conjunction with the accompanyingdrawings, in which:

[0015]FIG. 1 is a block diagram of a heteromorphic waveform function inaccordance with the present invention within a next generation (XG)appliqué;

[0016]FIG. 2 is an illustration of a frequency agile heteromorphicwaveform adapting to fill available time-frequency spectrum gaps;

[0017]FIG. 3 is an illustration of a heteromorphic waveform adapting tomultiple variables to optimize spectral efficiency;

[0018]FIG. 4 is a multi-carrier organization, signaling and multi-levelbandwidth-efficient coding and modulation for optimizing channelestimation data;

[0019]FIG. 5 is a representation of frequency/time/coding of aheteromorphic waveform in accordance with the present invention; and

[0020]FIG. 6 is a block diagram illustration of multi-levelconfiguration of LDPC-based coded modulation scheme to facilitate rapidadaptation of code parameters.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0021] The present invention is a heteromorphic waveform thatdynamically adapts in frequency, time, modulation, code, data rate,power, signaling, and multi-carrier organization. The waveform willincrease spectral efficiency by enabling efficient, opportunistic andcooperative spectrum use. It reacts to time-varying channel and useconditions by seizing time/frequency/spatial “holes” and using the mostefficient coding, modulation, signaling and multi-carrier organizationconsistent with non-interfering communications. The heteromorphicwaveform of the invention is subdivided into two major components asfollows:

[0022] Adaptive Multi-Carrier Organization and Signaling configures avariable carrier frequency and variable bandwidth signal into one ormany sub-carriers that are dynamically placed over a span of up to 250MHz to avoid or minimize interference with transmissions of existingspectrum users. Each sub-carrier is independently modulated by directsequence spread-spectrum (DS SS) for variable spreading and coding gainagainst cooperative, non-cooperative, and threat signals. A combinationtime/code pilot is embedded within the waveform to empower optimizationbased on sub-carrier channel estimates. The waveform supports a broadrange of adaptive/hybrid multiple access schemes including combinationsof CDMA, TDMA, FDMA, and FHMA.

[0023] Adaptive Multi-Level Bandwidth-Efficient Coding and Modulation(BECM) provides a family of BECM schemes, incorporating bothmulti-constellation modulation and forward error-correction coding. ALow Density Parity-Check Code (LDPC) coded modulation family will beused to advance the state-of-the-art in bandwidth efficiency andadaptation capability. Adapting the modulation constellation, code rate,and code length to match the available spectrum and sub-carrierconditions will maximize spectral efficiency while meeting quality ofservice (QoS) and data rate needs.

[0024] Overall spectral efficiency depends on a combination offrequency, space, and time efficiency of spectrum use. As these factorsare closely inter-dependent, improving efficiency in one area oftenreduces efficiency in another.

[0025] Decrease spectral use per call/connection

[0026] increase modulation efficiency (bits/sec/Hz)

[0027] improve error-correction coding efficiency

[0028] compress source information

[0029] use adaptive (i.e., hybrid) multiple-access technique with “soft”capacity limits (e.g., MC-CDMA where FDMA/CDMA is possible.

[0030] Increase spatial reuse of bandwidth

[0031] increase power efficiency of modulation (minimum E_(b)/N_(o) toachieve sufficient BER)

[0032] use fast-adaptation in power control

[0033] reduce sensitivity to interference by waveform design

[0034] transmit a more “interference-friendly” waveform

[0035] spread signal information over wider bandwidth

[0036] increase directional sharing of bandwidth

[0037] Increase temporal sharing of bandwidth

[0038] coordinate time use of spectrum (e.g., via multiple accesstechnique)

[0039] seize temporal “holes” in spectrum use as they become available(e.g., fast signal acquisition, burst-by-burst adaptation)

[0040] Many of these strategies conflict with each other—increasing themodulation efficiency decreases power efficiency. An accurate assessmentof overall spectral utilization efficiency requires consideration of thecomplex interaction of frequency/time/space reuse of the electromagneticspectrum.

[0041] Referring to FIG. 1, there is illustrated a heteromorphicwaveform function that dynamically “morphs” to fill unused spectrum“holes” to dramatically increase spectral utilization. Overall waveformadaptation can be considered a hierarchical combination of “internal”and “external” functions, features, and parameter sets that determinethe final transmitted waveform. The “external” set provides a definitionof the frequency and temporal opportunities, along with otherenvironment characteristics. The definition of the “internal” setmodifies how the waveform “reacts” within its overall bandwidth span toimplement strategies that optimize the waveform parameters for maximumspectrum efficiency consistent with local channel conditions, mutualinterference avoidance, and LPI/LPD requirements.

[0042] The waveform of the present invention is a multi-carrierdirect-sequence spread-spectrum (MC-DS SS), multi-rate,multi-constellation composite wideband waveform, quickly adaptable intime, frequency, power, modulation type, rate, code, multi-carrierorganization and access method. An adaptable interface will allow avariety of access and control techniques and will adapt to othernetworks in the same frequency allocation band and physical space, andto time-varying channel conditions, threats and user needs. The waveformuses available short-duration (milliseconds) time segments on a packetbasis, relinquishing channels to other networks as they become active,and seizing other channels based on predicted availability.

[0043] Frequency agility is achieved in several ways. First, the centerfrequency and RF bandwidth of the waveform can vary to occupy differentfrequency channels as the time usage of those channels varies. This isshown in FIG. 2, which is a representation of spectrum utilization forfour frequency channels as a function of time. The existing users areasindicate transmissions from existing non-XG users and the empty spectrumareas indicate “holes” in time-frequency spectrum use. Consider an XGtransmission as shown utilizing the first available “gap” on frequencychannel F1. At point A, the waveform demonstrates macroscopic frequencyagility by “morphing” its center frequency and bandwidth span to brieflyoccupy both frequency channels F1 and F2 before morphing again intochannel F2. At point B, both the non-XG and XG transmissions occupyfrequency channel F2. The non-XG transmissions occupy only a portion offrequency channel F2. Within the full bandwidth span of the XGtransmission, the waveform organizes its sub-carriers to occupy somesubset of the full span. Hence, the occupied bandwidth of the waveformwill be less than or equal to the full bandwidth span. This microscopicfrequency agility is used to avoid the portions of the frequency channeloccupied by the non-XG signals. No power, or power within an acceptableSIR value for the non-XG signals, is transmitted on these unusedsub-carriers in order to avoid interference with other transmissions.This combination of macroscopic and microscopic frequency agilitymaximizes XG spectral efficiency by seizing available gaps infrequency/space/time freeing up needed spectrum for both communicationsand sensor (such as radar) functions.

[0044] Referring to FIG. 3, there is shown a representation of thewaveform in 2-D on the left and in 3-D on the right. The legend in thecenter of the figure highlights the areas of unspread QAM-basedmodulation, empty spectrum, excluded spectrum, and DS-SS-basedmodulation. The excluded spectrum represents the combination oftime-frequency holes that are not available for waveform use as providedby externally controlled functions within the XG radio. The waveformdemonstrates microscopic frequency agility and organizes the signalenergy to avoid these exclusion zones, “morphing” dynamically to assumevaried shapes in 3-D (frequency, time, power). Note that the exclusionzones are displayed as “blocked out” in the 3-D representation; no poweris transmitted on those time-frequency combinations. On othersub-carriers, the waveform utilizes a combination of QAM-basedmodulation and both single carrier and multi-carrier direct-sequencespread spectrum coexisting in time on different frequency sub-channels,with time-varying modulation on a given sub-channel. Bandwidth-efficientcoding and modulation (BECM) schemes and sub-carrier organization arealso continuously adapted to maximize overall spectral utilizationefficiency. Based on signal optimization and data rate requirements, theXG waveform may choose to leave some of the available time-frequencyholes empty.

[0045] Construction of the waveform is partitioned into two majorfunctional components as described below.

[0046] Adaptive Multi-Carrier Organization and Signaling configures achannel of up to 250 MHz bandwidth span into one or many variable widthsub-carriers that are independently modulated by Direct-Sequence SpreadSpectrum (DS-SS) for variable coding gain. The waveform will support abroad range of multiple access techniques including CDMA, TDMA, FDMA,FHMA, CSMA/CA, and RTS/CTS. Multiple users are served simultaneously anduniquely at varying data rates on sub-channels contained within the upto 250 MHz bandwidth span.

[0047] Adaptive Multi-Level Bandwidth-Efficient Coding and Modulation(BECM) provides a family of BECM schemes, incorporating bothmulti-constellation modulation and multi-level forward error correctioncoding that is optimized for sub-channel conditions. The baseline designuses Lower Density Parity-Check Codes (LDPC), currently favored byrecent research in BECM, as the basis for coded modulation technology.

[0048] Adaptation in multiple dimensions is required in order to realizeimprovements in spectral efficiency by utilizing gaps infrequency/space/time. The heteromorphic waveform is simultaneouslyadaptive across many different dimensions as summarized in Table 1. Thecarrier frequency, bandwidth span, and occupied bandwidth are variedgiving the XG transmission the required macroscopic frequency agility to“hop” from channel to channel as needed. The adaptive multi-carrierorganization and signaling capability structures the up to 250 MHzbandwidth span into one or many variable width sub-carriers to supportmicroscopic frequency agility and avoid transmissions within thewaveform bandwidth. The resulting occupied bandwidth will depend on acombination of user data rate requirements, sub-channel conditions, andthe processing capability of the XG platform. Adaptive multi-levelbandwidth-efficient coding and modulation (BECM) takes advantage of XGchannel estimation enabled by pilot symbol elements embedded within thewaveform to select error-correction codes and modulation constellationsthat optimize capacity across the sub-channels. In addition to powercontrol schemes used to minimize multiple access interference, thewaveform has a burst-by-burst “fast-adapt” power control capability torapidly relinquish use of an individual sub-carrier or the entireoccupied bandwidth, as indicated by an external control signal inresponse to detection of non-XG signals coincident intime/frequency/space. TABLE The Heteromorphic Waveform simultaneouslyadapts in multiple dimensions to increase spectral utilizationefficiency. Adaptation Capability Motivation Discussion CarrierFrequency Macroscopic Allows use of frequency Frequency Agilityspace/time “gaps: across full band of operation Bandwidth SpanMacroscopic Allows use of different Frequency Agility widthfrequency/space/ time “gaps” Sub-carrier Organization Microscopic Avoidsinterference and and Signaling Frequency Agility jammers (OccupiedBandwidth)) Sub-carrier Bandwidth- Sub-channel Matches XG capacity toEfficient Coding and Optimized Data channel conditions Modulation (BECM)Rates Fast-Adapt Power Control Power Efficiency Promotes spatial reuseby reducing interference to other users Fast Acquisition/Pilot RapidAllows use of short Symbols Seize/Release and frequency/space/timeChannel “gaps” Estimation

[0049] Referring to FIG. 4, there is illustrated a waveform adaptationfunction residing in an XG radio. The adaptive multi-carrierorganization and signaling section defines the preamble and pilotsymbols, assigns sub-carrier placement and capacity, and applies anyneeded PN spreading, time diversity, and channelization to the userdata. The adaptive multi-level bandwidth-efficient coding and modulationsection codes and maps the coded data to the assigned sub-carriers. Thesignal is then adaptively power controlled resulting in the completeheteromorphic waveform bandwidth spanning up to 250 MHz. Channelestimation on the received data is performed by using the bi-directionalpilot symbols embedded in the waveform for each transmission to estimatethe widely varying sub-carrier channel characteristics between any pairof XG nodes. A decoded preamble contains channel estimate informationfrom the other end of the link. Channel estimation data are passed toeach adaptation block to optimize sub-carrier capacity. In this way, thechannel estimates drive the adaptation of the multi-carrier organizationand signaling and multi-level bandwidth-efficient coding and modulation.The pilot symbol design for channel estimation is discussed later.

[0050] The multi-carrier structure of the heteromorphic waveform allowsspatial processing technology to be applied independently across thedifferent sub-bands. Hence, the waveform will not only be compatiblewith current and future spatial processing, but will enable performanceimprovements compared with techniques that yield one solution for thefull bandwidth. This includes both beam and null forming and space/pathdiversity processing systems, leveraging the enhanced interferencesuppression and higher data rate transmission gains achieved acrossmultiple technology areas to increase spectral efficiency.

[0051] Referring to FIG. 5, there is illustrated multiple 3-Dfrequency/time/power representations of the waveform. The x-y plane inthe far left picture shows a time-frequency mapping of the waveform.User data are mapped across up to K multiple variable-widthsub-carriers. Multiple sub-carriers can be aggregated to form variablewidth sub-bands within the total RF bandwidth. An FFT-basedimplementation is utilized with variable length integration time. Thepower level of each sub-carrier as a function of frequency and time canbe made arbitrarily small to avoid overlapping with other transmissionsin the environment. The waveform simultaneously supports multiplespreading widths and modulation formats on different sub-carriers.

[0052] The top illustration in FIG. 5 shows a notional view of one waythe waveform supports multiple users through CDMA. In the illustration,one sub-carrier is dedicated to a single user by assigning a single,shorter PN spreading code to that user to increase the data rate, whileon the other sub-carrier, multiple variable rate users access thechannel with different length PN codes. This is further shown in theillustration labeled “CDMA Mode” with the power of the codes of usersC_(A), C_(B), and C_(C) combining to form the aggregate power.Alternatively, one user can concentrate its data to occupy an entiresub-carrier using PSK/QAM-based modulation. The waveform also supports ahybrid mode where different portions of the user data are coded intodifferent modulation formats as shown in the lower right comer of FIG.5. Consider a non-XG transmission occupying the upper and lower portionsof the frequency band.

[0053] Based on channel estimates provided by the waveform, thetransmission shown is then mapped into two parts. Part 1 spreads theuser data across the entire bandwidth in order to reduce the powerspectral density below a level that is harmful to the non-XGtransmission; part 2 concentrates the remaining data in the unoccupiedbandwidth.

[0054] Across the bandwidth of a wideband signal, some frequencies willexperience strong channel gains, while others experience deep fades.Both single carrier and MC-DS SS provides against narrowbandinterference and time-varying frequency-selective fading caused by themultipath propagation of the radio channel. For the single carrier case,when the bandwidth of a carrier exceeds the coherence bandwidth (B_(C))of the channel, multiple rake receiver “fingers” are needed to resolvethe individual multipath components and capture the achievable diversitygain. The number of components that can be resolved, and hence, thenumber of rake receivers needed, is the ratio of the carrier bandwidthto the coherence bandwidth. An alternate approach is to divide the totalbandwidth B into N multiple sub-carriers of narrower bandwidth b=B/N,each roughly equal to the coherence bandwidth (b≈B_(C)). With multiplecarriers, the frequency diversity of the original wide bandwidth isretained by diversity combining the multiple independent carriers in thefrequency domain instead of the multiple rake fingers of the singlecarrier in the time domain. The amount of frequency diversity gain canbe traded against data rate in this type of waveform design bytransmitting a given data symbol across multiple sub-carriers (i.e.,spread in frequency) and combining the test statistics from thosesub-carriers before making a final decision on the data. In the limit,as each sub-carrier is modulated by data independent of the othersub-carriers, the overall transmission rate is maximized, and eachsymbol is sent without frequency diversity.

[0055] It has been shown that the performance of a single carrier DS SSwaveform with a rake receiver and an equivalent designed MC-DS SSwaveform are similar.

[0056] When the available bandwidth (and the data rate) is much greaterthan the coherence bandwidth, then a large number of rake fingers areneeded, significantly increasing receiver complexity. Instead of N(=B/B_(C)) fingers each processing a signal of bandwidth B for singlecarrier DS SS, the MC-DS SS waveform requires N fingers (one persub-carrier) each processing a signal of bandwidth b (=B/N) resulting ina reduced complexity receiver. This occurs because the chip duration onthe sub-carriers is M times longer than that of the single carriersystem, reducing the number of computations needed to successfullydemodulate the signal. When more than three to four rake fingers areneeded, multi-carrier implementations are more efficient.

[0057] The implementation advantage of multi-carrier modulation isfurther highlighted when narrowband interferers are present since amulti-carrier system does not require a continuous frequency band. Forapplication in XG systems, the multiple carriers are overlaid upon anexisting set of narrowband signals simply by leaving appropriate gaps inthe placement of the multiple sub-carriers. This adaptive “re-routing”of sub-carrier placement to avoid the interferers can be accomplishedwithout performance loss relative to contiguous sub-carriers with thesame total occupied bandwidth. A single-carrier signal must implementadaptive notch filters whose achievable notch depth and notch bandwidthare related in complexity.

[0058] An advantage of MC-DS SS waveform flexibility is using differentdata rates in some or all of the sub-carriers in order to send more dataon “strong” sub-carriers while sending less data on “weak” ones. Theability to capitalize on this flexibility depends upon how accuratelythe system estimates the state of the fade on the differentsub-carriers. The pilot performs this channel estimation where theability to accurately estimate the fade depends upon many systemparameters including signal-to-noise ratio (SNR), signal-to-interferenceratio (SIR), Doppler spread, and forward error correction.

[0059] The waveform of the invention incorporates a channel estimationcapability to guide adaptation of multi-carrier organization andsignaling and bandwidth-efficient coding and modulation on a sub-carrierbasis to optimize spectral utilization efficiency. The basis for channelestimation is a hybrid CDMA/TDMA pilot that consists of a spreading codeembedded within the preamble of a data burst. These pilot symbols arelogically equivalent to a training sequence for an adaptive equalizer.Use of a pilot allows coherent demodulation, improving power efficiency.Spreading the pilot reduces the probability of detection and intercept.Anti-jam resistance is provided by making sure that the pilot is spreadat least as much as the data so that a jammer could not easily defeatthe waveform by focusing efforts solely on the pilot.

[0060] The use of a pilot also provides a “snapshot” of sub-carrierfading that can be used to estimate the coherence bandwidth of thechannel. This estimate is used as the basis for adapting sub-carrierwidth and placement subject to spectrum gap availability constraints.Just as sub-carrier width is driven by the coherence bandwidth of thechannel, the rate of change of the fading is driven by the coherencetime of the channel. The coherence time provides a measure of how longthe channel estimates remain valid, and is inversely proportional toDoppler shift. For example, a vehicle moving at 50 mph and communicatingon a frequency of 2.5 GHz has a Doppler shift of 186 Hz, indicating thatchannel estimates and subsequent multivariate adaptation will need to beupdated on the order of every 5.4 ms. Data will be sent at the same rateon each sub-carrier when channel estimates are either not available, orwhose ages have exceeded the coherence time of the channel.

[0061] Using the MC signal as the basis for multi-carrier organizationand signaling gives a wide array of design trade-offs to maximizespectral efficiency. Multiple combinations of different waveformparameters provide an equivalent user payload data rate. Theeffectiveness of adaptation in multiple variables includes thefollowing:

[0062] Variable bandwidth: varying the bandwidth span and occupiedbandwidth allows the waveform to match the bandwidth available. Widerbandwidths provide a greater amount of raw capacity that can be tradedfor diversity, coding, spreading gain, etc. Narrower bandwidths providea structure that allows waveform operation when small amounts ofspectrum are available.

[0063] Variable number of sub-carriers: by varying the number ofsub-carriers, the available bandwidth can be organized to avoidnarrowband interference/jamming on select sub-carriers. If just onesub-carrier is used, the waveform “morphs” into a single carrierwaveform (e.g., DS SS, conventional QPSK, etc.).

[0064] Variable sub-carrier organization: mapping user data intodifferent combinations of sub-carriers allows different types of systemgain to be applied to the signal to combat fading and interference.Spreading gain and frequency diversity gain can be applied acrossadjacent sub-carriers, and varying amounts of interference averaging canbe achieved by mapping the data across non-contiguous sub-carriers.

[0065] Variable sub-carrier data rate: by monitoring the state of eachsub-carrier, and using higher order modulation where channel conditionsallow, the data rate within each sub-carrier can be optimized.

[0066] Variable frequency diversity: by transmitting multiple bits inparallel on different sub-carriers (multi-carrier load sharing), datarate is traded for frequency diversity.

[0067] Because of the severe sensitivity of any DS SS system to thenear-far problem, one or more means of mitigation must be part of thesystem design. For ad-hoc wireless systems, the commercial cellular CDMAsolution of base-station-oriented power control requires centralizedcontrol of all the transmitters. Alternatives to enhancing the waveformto near-far interference includes the following:

[0068] The XG capability of “morphing” the signal infrequency/space/time itself provides some inherent resistance tonear-far interference. Adaptation strategies for multi-carrierorganization and signaling and bandwidth-efficient coding and modulationconsider the effects of near-far multiple access interference (MAI).

[0069] To seize and release spectrum opportunities, the data areorganized into variable-length packets. This leads naturally into theability to multiplex users based on packet arrival time. Hence, TDMA canbe supported by the waveform for ad-hoc mobile networking.

[0070] Sub-carrier slots can be arranged to support FHMA with anear-orthogonal frequency-hopping (FH) pattern so that near-far signalstypically occupy different sub-carriers at any instant in time.

[0071] Within the ad-hoc network, clusters of users arrange themselvesin sub-networks, improving the effectiveness of standard power control.

[0072] When LPI is not required, a single-user MAI suppression techniquebased upon a receiver designed to minimize mean-square error can beemployed. Such a receiver is well-suited for an ad-hoc network, since itdoes not require apriori knowledge of the parameters of any of the usersin the system. However, short spreading sequences (i.e., those whoseperiod equals the duration of a data symbol) are used.

[0073] When available, spatial processing provides additional near-farresistance with appropriate beamforming. In particular, sub-bandbeamforming is anticipated to provide greater amounts of near-farinterference suppression.

[0074] The heteromorphic waveform described herein allows solution ofthe near-far problem through a combination of several techniques ofadaptive frequency and/or time allocation, frequency hopping, powercontrol or spatial arrays. Thus, the waveform will be compatible withTDMA, TSMA, FDMA, CDMA, FHMA and other commonly used supplementalcontrol techniques such as CSMA/CA and RTS/CTS. For integration as anappliqué solution, the waveform utilizes the multiple access scheme ofthe base radio system if necessary, or adapt it if allowed. Hybridmultiple access schemes can be used that dynamically match the multipleaccess format to the local spectrum utilization characteristics leadingto even further increases in spectrum utilization.

[0075] Error-correction codes are well known to provide significantlyincreased power efficiency for a small (or no) reduction in bandwidthefficiency at the expense of increased complexity. The baselineerror-correction coding and modulation design is based on an adaptivelow-density parity-check coded (LDPC) modulation code family that iswell-suited for use in XG systems.

[0076] LDPC codes are linear binary block codes whose parity-checkmatrix H possesses a low density of ones (i.e., it consists mostly ofzeros). These characteristics endow the codes with an improved weightspectrum and a simple near-optimum decoding algorithm. The decodingalgorithm is iterative, much like the trellis-turbo decoding algorithm,but the LDPC algorithm iterates over a graph rather than between twotrellises. Note that although for TTCM the two trellises can be put ingraph form, the graph is much more complex than the LDPC graph. Thedescribed LDPC modulation family enables fast adaptation by thefollowing techniques:

[0077] The use of a multi-level encoding structure, which is a naturalarchitecture for multi-rate coding

[0078] Simple component encoder implementation via simple shift-registercircuits through the use of cyclic and quasi-cyclic LDPC codes

[0079] The heteromorphic waveform of the present invention willincorporate a range of code lengths and code rates to optimizeperformance based on spectrum availability and sub-carrier channelconditions.

[0080] Referring to FIG. 6, there is illustrated binary LDPC codesarranged in a multi-level configuration, consisting of N component codesand a mapper (modulator). By this approach, the bandwidth efficiency(and bandwidth) can be widely varied by varying the code rates of thecomponent codes and/or the constellation size of the mapper. Thismulti-level configuration gives near-capacity performance. The number oflevels is generally matched to the constellation size. For a 2^(N)-aryconstellation, there will be N encoders.

[0081] Encoders for cyclic LDPC codes can be constructed using thewell-known shift-register circuits used to encode BCH codes. The nominalcodeword length is n and the nominal data word length is k giving anominal code rate of k/n with these parameters easily modified.Low-latency adaptation will need a range of code lengths.

[0082] Using LDPC code families as the basis for bandwidth-efficientcoding and modulation gives a wide array of trade-offs to maximizespectral efficiency. Multiple combinations of modulation constellationand code rate provide an equivalent user payload data rate, and codelength will also impact error performance. The effectiveness ofadaptation in multiple variables includes the following:

[0083] Modulation constellation: varying the modulation constellationprovides the capability to trade-off raw data (rate for powerefficiency. Small modulation constellations allow operation at lowerreceive power levels to extend coverage range. Larger modulationconstellations (up to 64 QAM) give larger raw capacity that can then betraded for coding gain to match sub-carrier channel conditions.

[0084] Code rate: varying code rate provides an additional degree offreedom to match code strength to local channel conditions. Low ratecodes help extend link margin and high rate codes will deliver anappropriate amount of coding gain while maximizing user data rate.

[0085] Code length: variable code lengths are needed to efficiently mapuser data into a wide range of sub-carrier capacities. Long codes willbe used to operate near the capacity limit when long temporal “gaps” inspectrum are available. Short codes will be used to meet low latencyrequirements, provide fast adaptation, and allow the waveform to seizeshort/small spectrum gaps.

[0086] Multi-level coding: using multi-level coding simplifies thecoding and decoding architecture and is a natural fit for supportingadaptive coding strategies by “pre-filling” multiple user data blocks sothat it is ready for immediate transmission once channel estimation dataare available to guide code selection.

[0087] The combination of a wideband MC-DS SS waveform structure thatcan dynamically change carrier frequency, bandwidth, and sub-carrierorganization and signaling with bandwidth-efficient coding andmodulation is used to create a heteromorphic waveform. Waveformarchitecture has been structured to innovate beyond the currentstate-of-the-art in wireless communications and information theoryresearch. This invention extends the boundaries by adapting to fillavailable spectrum “holes” and optimizing user data rate on availablesub-carriers using simultaneous multivariate adaptation of waveformparameters.

[0088] Although a preferred embodiment of the invention has beenillustrated in the accompanying drawings and described in the foregoingdescription, it will be understood that the invention is not limited tothe embodiments disclosed, but is capable of numerous rearrangements andmodifications of parts and elements without departing from the spirit ofthe invention.

What is claimed is:
 1. A system for generating an adaptive air interfacewaveform, the system comprising: an adaptive multi-carrier organizationand signaling component operable to generate a waveform comprising avariable carrier frequency and variable bandwidth signal that comprisesone or more subcarriers that are dynamically placeable over a range offrequencies, each subcarrier being separately modulated according to adirect sequence (DS) spread spectrum (SS) technique, the waveform havingan embedded pilot usable to optimize one or more spectrum efficienciesof the waveform; and an adaptive multi-level bandwidth-efficient codingand modulation (BECM) component operable to adapt a modulationconstellation, a code rate, and a code length of the generated waveformaccording to an available spectrum and one or more sub-carrierconditions.
 2. The system of claim 1, wherein the generated waveform isa heteromorphic waveform operable to dynamically adapt with respect toone or more of frequency, time, modulation, code, data rate, power,signaling, and multi-carrier organization.
 3. The system of claim 1,wherein the range of frequencies spans approximately 250 MHz.
 4. Thesystem of claim 1, wherein the generated waveform is operable to use oneor more unused holes in a spectrum defined by one or more of frequency,space, and time.
 5. The system of claim 1, wherein the generatedwaveform supports a plurality of multiple access (MA) techniques.
 6. Thesystem of claim 5, wherein the plurality of MA techniques comprise: oneor more carrier division multiple access (CDMA) techniques; one or moretime division multiple access (TDMA) techniques; one or more frequencydivision multiple access (FDMA) techniques; one or more frequency-hoppedmultiple access (FHMA) techniques;
 7. The system of claim 5, wherein atleast one of the MA techniques is a hybrid MA technique.
 8. The systemof claim 1, wherein the BECM uses a low-density parity-check (LDPC) codemodule technique to adapt a modulation constellation, a code rate, and acode length of the generated waveform.
 9. The system of claim 1, whereinthe BECM is operable to adapt a modulation constellation, a code rate,and a code length of the generated waveform according to one or morequality of service (QoS) requirements and one or more data raterequirements, in addition to an available spectrum and one or moresub-carrier conditions.
 10. The system of claim 1, wherein the generatedwaveform exhibits both macroscopic frequency agility and microscopicfrequency agility.
 11. A method for generating an adaptive air interfacewaveform, the method comprising: generating a waveform comprising avariable carrier frequency and variable bandwidth signal that comprisesone or more subcarriers that are dynamically placeable over a range offrequencies, each subcarrier being separately modulated according to adirect sequence (DS) spread spectrum (SS) technique, the waveform havingan embedded pilot usable to optimize one or more spectrum efficienciesof the waveform; and adapting a modulation constellation, a code rate,and a code length of the generated waveform according to an availablespectrum and one or more sub-carrier conditions.
 12. The method of claim11, wherein the generated waveform is a heteromorphic waveform operableto dynamically adapt with respect to one or more of frequency, time,modulation, code, data rate, power, signaling, and multi-carrierorganization.
 13. The method of claim 11, wherein the range offrequencies spans approximately 250 MHz.
 14. The method of claim 11,wherein the generated waveform is operable to use one or more unusedholes in a spectrum defined by one or more of frequency, space, andtime.
 15. The method of claim 11, wherein the generated waveformsupports a plurality of multiple access (MA) techniques.
 16. The methodof claim 15, wherein the plurality of MA techniques comprise: one ormore carrier division multiple access (CDMA) techniques; one or moretime division multiple access (TDMA) techniques; one or more frequencydivision multiple access (FDMA) techniques; one or more frequency-hoppedmultiple access (FHMA) techniques;
 17. The method of claim 15, whereinat least one of the MA techniques is a hybrid MA technique.
 18. Themethod of claim 11, wherein a low-density parity-check (LDPC) codemodule technique is used to adapt a modulation constellation, a coderate, and a code length of the generated waveform.
 19. The method ofclaim 11, wherein the modulation constellation, code rate, and codelength of the generated waveform is adapted according to one or morequality of service (QoS) requirements and one or more data raterequirements, in addition to an available spectrum and one or moresub-carrier conditions.
 20. The method of claim 11, wherein thegenerated waveform exhibits both macroscopic frequency agility andmicroscopic frequency agility.
 21. Software for generating an adaptiveair interface waveform, the software embodied in media and when executedoperable to: generate a waveform comprising a variable carrier frequencyand variable bandwidth signal that comprises one or more subcarriersthat are dynamically placeable over a range of frequencies, eachsubcarrier being separately modulated according to a direct sequence(DS) spread spectrum (SS) technique, the waveform having an embeddedpilot usable to optimize one or more spectrum efficiencies of thewaveform; and adapt a modulation constellation, a code rate, and a codelength of the generated waveform according to an available spectrum andone or more sub-carrier conditions.
 22. The software of claim 21,wherein the generated waveform is a heteromorphic waveform operable todynamically adapt with respect to one or more of frequency, time,modulation, code, data rate, power, signaling, and multi-carrierorganization.
 23. The software of claim 21, wherein the range offrequencies spans approximately 250 MHz.
 24. The software of claim 21,wherein the generated waveform is operable to use one or more unusedholes in a spectrum defined by one or more of frequency, space, andtime.
 25. The software of claim 21, wherein the generated waveformsupports a plurality of multiple access (MA) techniques.
 26. Thesoftware of claim 25, wherein the plurality of MA techniques comprise:one or more carrier division multiple access (CDMA) techniques; one ormore time division multiple access (TDMA) techniques; one or morefrequency division multiple access (FDMA) techniques; one or morefrequency-hopped multiple access (FHMA) techniques;
 27. The software ofclaim 25, wherein at least one of the MA techniques is a hybrid MAtechnique.
 28. The software of claim 21, wherein a low-densityparity-check (LDPC) code module technique is used to adapt a modulationconstellation, a code rate, and a code length of the generated waveform.29. The software of claim 21, wherein the modulation constellation, coderate, and code length of the generated waveform is adapted according toone or more quality of service (QoS) requirements and one or more datarate requirements, in addition to an available spectrum and one or moresub-carrier conditions.
 30. The software of claim 21, wherein thegenerated waveform exhibits both macroscopic frequency agility andmicroscopic frequency agility.
 31. A system for generating an adaptiveair interface waveform, the system comprising: means for generating awaveform comprising a variable carrier frequency and variable bandwidthsignal that comprises one or more subcarriers that are dynamicallyplaceable over a range of frequencies, each subcarrier being separatelymodulated according to a direct sequence (DS) spread spectrum (SS)technique, the waveform having an embedded pilot usable to optimize oneor more spectrum efficiencies of the waveform; and means for adapting amodulation constellation, a code rate, and a code length of thegenerated waveform according to an available spectrum and one or moresub-carrier conditions.